Conventionally, a frequency synthesizer of this kind is formed of a PLL comprising an oscillator using a quartz-crystal oscillator, a phase comparator, a divider (programmable one), a VCO (voltage controlled oscillator), etc. Such a frequency synthesizer generates and outputs an oscillation signal of a frequency variable by constant spacings by locking the VCO on the basis of the phase difference between a reference oscillation signal from the oscillator and the output oscillation signal and using the divider.
FIG. 1 is a block diagram showing an arrangement of a radio transmitter using a prior art frequency synthesizer. In FIG. 1, the radio transmitter comprises a frequency synthesizer 1 for supplying an stable output oscillation signal SA with a frequency variable by constant spacings, a quadrature modulator 2 for applying QPSK (quadrature phase shift keying) or QAM (quadrature amplitude modulation) to the output oscillation signal SA by using phase angle modulation signals (I and Q signals), a power amplifier 3 for amplifying the modulated high frequency signal SM from the quadrature modulator 2 for transmission, and an antenna 4 for radiating the high frequency electric power SP from the power amplifier 3 as radio wave. Also, the frequency synthesizer 1 is provided with a reference signal oscillator 5 for oscillating a reference signal, and a phase comparator 6 for making a phase comparison between a frequency-divided signal and the reference oscillation signal from the reference signal oscillator 5. And, it is also provided with a low pass filter (LPF) 7 for removing undesirable harmonics and noise from a phase error signal from the phase comparator 6 and supplying a DC voltage, a voltage controlled oscillator (VCO) 8 for sending out an output oscillation signal SA with its frequency locked by the DC voltage from the LPF 7, and a frequency divider 9 for frequency-dividing the output oscillation signal so as to provide a frequency-divided signal which is frequency-variable by constant spacings and for supplying the frequency-divided signal to the phase comparator 6.
Here, we discuss operation of the radio transmitter or a prior art example, in the following.
The phase comparator 6 in the frequency synthesizer 1 compares the phases of the reference oscillation signal from the reference signal oscillator 5 and the frequency-divided signal from the frequency divider 9. The phase error signal obtained from the comparison is passed through an LPF 7, which removes undesirable harmonics and noise from the phase error signal from the phase comparator 6 to yield a DC voltage, that is, a frequency controlling signal, which is applied to the control terminal of a VCO 8. It is noted that the amplitude and phase characteristics of the LPF 7 determines the response and synchronization characteristics of the PLL in this frequency synthesizer 1. That is, selecting the inductance and capacitance of the LPF is to set a time characteristic of the frequency divider 9 with respect to frequency shift. The frequency synthesizer 1 supplies a stable output oscillation signal SA with a frequency variable by constant spacings to the quadrature modulator 2, which applies modulation such as (QPSK and (QAM with phase angle modulation signals (I and Q signals) to the output oscillation signal SA for output. The modulated radio signal SM is power amplified by the power amplifier 3 to radiate the amplified radio signal power SP through the antenna 4 as radio wave.
In such transmitter, the radio signal power SP from the power amplifier 3 is relatively large. For this reason, in order to prevent undesirable radiation leaking from the radio signal power from the power amplifier 3 and radio wave radiated from the antenna 4 from going round into the other circuit including the frequency synthesizer 1, these circuits are shielded with a metal shield case. In spite of the shield, undesirable radiation and/or radiated radio wave may go into the resonator such as a dielectric resonant element which determines the oscillation frequency in the VCO 8 in the frequency synthesizer 1. This will cause the oscillation signal of the VCO 8 to be phase modulated, resulting in variation in the resonance frequency. Accordingly, the error rate in demodulating in a receiver the radio wave which has been modulated in QPSK by the quadrature modulator 2 will become high.
FIG. 2 is a phase plane diagram defined by I and Q axes on which what the modulated radio signal SM is demodulated into in a receiver is plotted, while FIG. 3 is a phase plane diagram defined by I and Q axes on which what the modulated radio signal SM in case of variations in the resonance frequency is demodulated into in a receiver is plotted. In FIG. 2, the demodulated version of the modulated radio signal SM in the ideal state without variation in the resonance frequency is shown as 4 phases (4 points) on an IQ plane. In this ideal state, the points plotted on the IQ plane are sufficiently far from one another, yielding a low error rate of demodulated data in a receiver.
On the other hand, occurrence of variation in the modulated radio signal SM which has been affected by the variation in the resonance frequency, that is, occurrence of frequency variation in the output oscillation signal SA from the frequency synthesizer 1 would cause the phase of the output oscillation signal SA to change, resulting in each of demodulated data of this case, when plotted on the IQ plane, spreading on the circumference of a circle as shown in FIG. 3. In this case, demodulated data may even go across the I or Q axis, which makes very high the error rate of demodulated data in the receiver. Even when demodulated data do not go so far as to cross the I or Q axis, a variation in the phase in demodulation may cause the error rate of demodulated data to rise.
Though such variation in the frequency of the output oscillation signal SA from the frequency synthesizer 1 is corrected by PLL operation, since the modulation is performed in the quadrature modulator 2, the phase and the amplitude vary with time at a rate responsive to the frequency of the phase angle modulated signal. Further, for the purpose of more efficient utilization of frequency, the modulation tends to be made at a higher rate. Thus, the frequency of the output oscillation signal SA varies with time so fast that the frequency variation of the output oscillation signal SA from the frequency synthesizer 1 can not be often suppressed by PLL operation. Radio communication devices have been proposed which are so arranged, in order to improve the frequency variation, that the transmission frequency and the oscillation frequencies of the frequency synthesizers are different.
FIG. 4 is a block diagram showing an arrangement of such a radio communication device wherein the transmission frequency and the oscillation frequencies of the frequency synthesizers are different. In FIG. 4, the device comprises two frequency synthesizers 10a and 10b, and a frequency mixer 11 for applying an addition or a subtraction to two output oscillation signals SC and SD from the frequency synthesizers 10a and 10b to yield sum frequencies or difference frequencies, respectively, that is, to output a stable output oscillation signal SE with a frequency variable by constant spacings. The radio communication device further comprises a quadrature modulator 12 for applying QPSK (quadrature phase shift keying) or QAM (quadrature amplitude modulation) to the output oscillation signal SE by using phase angle modulation signals (I and Q signals), a power amplifier 13 for amplifying the modulated high frequency signal SM for transmission, and an antenna 14 for radiating the high frequency electric power SP from the power amplifier 13 as radio wave.
Now, we discuss operation of the prior art device.
The output oscillation signals SC and SD from the two frequency synthesizers 10a and 10b are mixed by the frequency mixer 11 to output the stable output oscillation signal of either sum frequencies or difference frequencies which are different from one another by a constant frequency. The output oscillation signal SE is applied together with phase angle modulation signals (I and Q signals) to the quadrature modulator 12 and thereby modulated in QPSK or QAM to produce a modulated radio signal SM, which in turn is power amplified by the power amplifier 13. The resultant radio signal power SP is radiated through the antenna 14 as radio wave. Since the frequency of the radio signal power SP and the frequencies of the output oscillation signals SC and SD are different from one another in this operation, it is possible to avoid disturbance caused in case when spurious leaking from the radio signal power SP from the power amplifier 13 and the radio wave radiated from the antenna 14 go around into the VCO's. Thus, the frequency variation of the VCO's in the frequency synthesizers 10a and 10b is reduced.
Since this arrangement requires two frequency synthesizers 10a and 10b, the structure of the circuit becomes complicated. That is, the number of components increases arresting the reduction in the size of the device, which is especially disadvantageous for application in a portable wireless equipment. Producing sum or difference frequencies by using the frequency mixer 11 makes harmonic spurious apt to be generated, causing the generated spurious to mix in the receiving part, which may disable some receiving channel from receiving. In this case, the elimination of spurious component requires a band pass filter, above all, one with a sharp skirt characteristic, leading to need for complex operation. A radio communication equipment in which the output oscillation signal from a frequency synthesizer is frequency multiplied for improvement is well known (See, for example, Japanese Utility Model Application Disclosure No. Hei2-1930 disclosed in 1990).
FIG. 5 is a block diagram showing an arrangement of a radio communication equipment in which the output oscillation signal from a conventional frequency synthesizer is frequency multiplied. In FIG. 5, the equipment comprising a frequency synthesizer 16 for supplying an output oscillation signal SA, a frequency multiplier 17 for frequency multiplying the output oscillation signal SA from the frequency synthesizer 16, a quadrature modulator 18 for modulating the output oscillation signal SE in (QPSK or QAM by using phase angle modulation signals (I and Q signals), a power amplifier 19 for amplifying the modulated radio signal SM for transmission, and an antenna 20 for radiating the high frequency electric power SP from the power amplifier 19 as radio wave.
Now, we discuss operation of the conventional radio equipment in the following.
The output oscillation signal SA from the frequency synthesizer 16 is frequency multiplied by the frequency multiplier 17 and modulated by the quadrature modulator 18. The modulated radio signal SM is amplified in the power amplifier 19 to produce an amplified radio power SP, which is radiated through the antenna 20 as radio wave. Since the frequency of the radio power SP, the transmission frequency, differs from that of the output oscillation signal SA from the frequency synthesizer 16 in this case, if spurious leaking from the radio power SP from the power amplifier 19 or radio wave radiated from the antenna 20 goes around into the VCO in the frequency synthesizer 16, variation in the frequency will not take place within the VCO in the frequency synthesizer 16.
However, in this radio communication equipment, the transfer function of the frequency synthesizer 16 is approximately linear, and so the quickness of shifting the frequency which can be represented by natural frequency has to be made identical regardless of whether the equipment is provided with a frequency multiplier 17 or not. That is, thus arranged radio communication equipment will use a frequency synthesizer capable of shift its frequency as quickly as the case when the equipment has no frequency multiplier 17 to make use of any necessary part of a characteristic curve for the linear transfer function thereof.
In the frequency synthesizer 16, constant-spacing radio channels is produced by incrementing or decrementing the divisor of the frequency divider at each step. The phase comparator in the frequency synthesizer 16 is suited to be integrated, and so is generally constructed by using logic IC's. Accordingly, an actual phase comparator shapes the waveform of input signal into a rectangular waveform and compares phases at the rising or falling edge of the waveform. That is, the phase error is detected once every cycle of the signal input to the phase comparator.
Thus, the natural frequency can not be set higher than the phase comparison frequency. In the frequency synthesizer 16, it is necessary to apply to the VCO, as a controlling voltage, the only components of the phase comparator output which are lower than the natural frequency by actually removing pulse components output every phase comparison cycle from the phase comparison output. Therefore, the natural frequency needs to be set sufficiently smaller than the phase comparison frequency. Furthermore, in consideration of the multiplication of output oscillation signal SA, the phase comparison frequency must be set 1/M of the radio channel spacing.
For this reason, if constant interval radio channels are to be realized, the upper bound of the realizable natural frequency of the frequency synthesizer 16 will become as low as 1/M of the upper bound of the realizable natural frequency of the frequency synthesizer in the arrangement shown in FIG. 1. On the other hand, in order for a frequency synthesizer to shift its frequency during a TDMA burst used in digital communication, the shift needs to be more quick. Thus, the arrangement shown in FIG. 5 copes with the frequency variation caused by disturbance of spurious leaking from radio power SP or radio wave radiated from the antenna 20 going around into the VCO, whereas it can not disadvantageously respond to a quick shift of radio channels.
These and other disadvantages of the prior art are overcome in accordance with the present invention. An object of the invention is to provide a frequency synthesizer which enables a quick shift of frequency channels without making larger the scale of the device and which does not require any specific frequency band characteristic of a band pass filter in extracting and dividing a signal of a predetermined band which has been multiplied and so permits the use of a widely used filter, resulting in a reduction in cost.